1. Field of the Invention
The present invention relates to an exhaust passage control valve which is used as a bypass valve to open and close a bypass passage of a silencer which is interposed in an exhaust system of an engine.
2. Description of Related Art
An exhaust passage control valve is provided with: a housing through which exhaust gases from an engine passes; a valve element mounted on the housing; and a valve spring for urging the valve element toward a closing side such that, when an exhaust-gas pressure has exceeded a predetermined value, the exhaust passage control valve is opened.
Conventionally, as an exhaust passage control valve, there has hitherto been ordinarily used a butterfly valve which is opened and closed by rotation of a valve element about a shaft. In this valve, bearings are required to support the shaft of the valve element, resulting in a higher cost. As a solution, there has been proposed the following valve in Japanese Published Unexamined Patent Application No. 287118/1999 as a low-cost exhaust passage control valve which does not require bearings. Namely, it employs a spiral valve element whose outer circumferential (or peripheral) portion is seated onto an annular valve seat portion of a housing of the valve. The exhaust passage control valve is thus opened and closed by extension and contraction of the valve element along an axis of spiral winding of the spiral valve element.
In this example, the housing has fixed thereto a spring receiving member in a posture to lie opposite to the opening direction of the valve element. Between this spring receiving member and the valve element, there is interposed a valve spring which is made up of a compression coil spring.
In the above-described exhaust passage control valve in which the valve element of a spiral shape is urged by the compression coil spring toward the closing side of the valve, there was a disadvantage in that vibrations between opening state and closing state of the valve element occurred when an exhaust-gas pressure is operated upon. In addition, in order to attain opening and closing characteristics of the valve in that, when the exhaust-gas pressure has reached a predetermined value, the valve is opened all at once or suddenly and, when the exhaust-gas pressure has become smaller than the predetermined value, the valve is closed all at once, it becomes necessary to increase a winding diameter and the number of winding to thereby set a spring constant at a low level. As a result, the valve spring becomes large in size and the minimizing the size of the exhaust passage control valve becomes difficult.
In view of the above-described points, the present invention has an object of providing an exhaust passage control valve in which the spring constant can be set at a low level without enlarging the valve spring and in which the vibrations between opening state and closing state of the valve element be effectively prevented.
In order to attain the above and other objects, the present invention is an exhaust passage control valve comprising: a housing through which exhaust gases from an engine pass; a valve element mounted on the housing; and a valve spring for urging the valve element toward a closing side such that, when an exhaust-gas pressure has exceeded a predetermined value, the exhaust passage control valve is opened. The valve element is constituted into a spiral valve element whose outer circumferential portion is seated onto an annular valve seat portion of the housing such that the exhaust passage control valve is opened and closed by extension and contraction of the valve element in a direction of an axis of spiralling. The valve spring is constituted by a helical torsion spring having a coiled cylindrical body portion, and an arm portion on each end of the coiled cylindrical body portion. The arm portions are engaged with a pair of spring receiving members so as to be slidable in a longitudinal direction of each of the arm portions, the pair of spring receiving members being provided in those portions of the housing which lie on circumferentially opposite sides of the valve seat portion to thereby urge the valve element through the coiled cylindrical body portion toward the closing side.
According to the present invention, the urging force in the direction of closing the valve element can be obtained mainly by the deflection reaction force of the arm portions on both sides of the helical torsion spring. The amount of deflection of the arm portion at an initial state is set so that a predetermined set load can be obtained. Here, if the coiled cylindrical body portion of the helical torsion spring is displaced in the opening direction of the valve element due to the extension of the valve element in the opening direction thereof, the arm portions slide relative to the spring receiving members. As a result, the effective length of the arm portions giving rise to the deflection force becomes large. Therefore, the deflection reaction force in the arm portion does not increase much, and the spring constant can be made low without increasing the size of the valve spring. In addition, accompanied by the movement of the valve element, there will occur a sliding friction in the arm portions with the spring receiving members, as well as a friction among the spirals of the coiled spring member in the coiled cylindrical body portion. The vibration dampening effect can thus be obtained due to the frictions, and the vibrations between opening state and closing state of the valve element can be effectively prevented.